1. Field of Invention
The present invention relates to the technical field of an electro-optical device such as a liquid-crystal display device. More particularly, the present invention relates to the technical field of an electro-optical device, such as a thin-film transistor (hereinafter referred as TFT) active-matrix liquid-crystal display device, which adopts an alternating drive method in which the polarities of the voltages applied to adjacent pixel electrodes are periodically alternated for every pixel row or every pixel column, so that the voltages applied to adjacent pixel electrodes in a row direction or in a column direction are inverted in polarity.
2. Description of Related Art
Electro-optical devices, such as liquid-crystal display devices, include an electro-optical material, such as a liquid crystal, interposed between a pair of substrates. The alignment state of the electro-optical material is controlled by the property of the electro-optical material and an alignment layer formed on the substrate on its surface facing the electro-optical material. If there is a step in the surface of the alignment layer (in other words, if there is a step in the surface of the pixel electrode beneath the alignment layer or in the surface of an interlayer insulator serving a substrate for the pixel electrode), an orientation defect (a disclination) occurs in the electro-optical material, depending on the magnitude of the step. If such an orientation defect occurs, proper driving of the electro-optical material in that portion becomes difficult, and the contrast ratio of the device drops due to a visible defect in the device. Since a TFT active-matrix electro-optical device includes, on a TFT array substrate, TFTs in many locations thereof for controlling and switching a variety of lines such as scanning lines, data lines, and capacitive lines, and pixel electrodes, a step inevitably occurs in the surface of an alignment layer in accordance with the presence of the lines and elements, if no planarizing process is performed.
Conventionally, the portion of the substrate suffering from such a step is aligned with the spacing between adjacent pixel electrodes, and a light-shielding layer called a black mask or a black matrix covers the portion of the step (i.e., the spacing between the pixel electrodes) so that the portion of the electro-optical material suffering from the orientation defect may remain hidden or may not contribute to display light.
Techniques for planarizing the surface of a substrate beneath the pixel electrode has been developed, in which an interlayer insulator beneath the pixel electrode is fabricated of a planarized film, such as an organic SOG (Spin On Glass) film, so that a step resulting from the presence of lines and TFTs may not be created.
The electro-optical device of this sort typically adopts an alternating drive method in which the polarity of a potential applied to the pixel electrodes is alternated at a predetermined pattern to prevent degradation of the electro-optical material as a result of the application of a direct current voltage and to control a cross-talk and flickering of a display screen image. A 1H alternating drive method is relatively easy to control and presents a high-quality image display, wherein during the presentation of a video signal of one frame or one field, the pixel electrodes arranged on an odd row are driven by a positive polarity relative to the potential of an opposing electrode, while the pixel electrodes arranged on an even row are driven by an negative polarity relative to the potential of the opposing electrode, and during the presentation of a video signal of a next frame or a next field, conversely, the pixel electrodes arranged on the even row are driven by a positive polarity while the pixel electrodes arranged on the odd row are driven by a negative polarity (in other words, the pixel electrodes on the same row are driven by the same polarity potential and the potential polarity is alternated every row with the period of frame or field). A 1S alternating drive method is also easy to control and presents a high-quality image display, wherein the pixel electrodes on the same column are driven by the same polarity potential while the potential polarity is alternated every column with the period of frame or field.